1. Field of Invention
The present invention relates to a method of monitoring an exposure system. More particularly, the present invention relates to a method of monitoring the conditions of a deep ultraviolet exposure system to be used in the manufacturing of semiconductor products.
2. Description of Related Art
To increase the operating speed of electronic equipment, dimensions of semiconductor devices in an integrated circuit are gradually reduced. Recently, the production of devices having a feature size in the submicron range is possible. However, due to the reduction of some critical dimensions, alignment of various circuit layers becomes an important factor in obtaining high-quality semiconductor products. If there is any misalignment between one layer and the next, circuits are no longer properly connected and the entire wafer chip may have to be discarded.
At present, most wafer manufacturing facilities rely heavily on deep ultraviolet (DUV) exposure systems for multi-layer alignment. Since a deep ultraviolet ray has a short wavelength, very accurate alignment can be obtained.
Because a deep ultraviolet exposure station is a very expensive piece of equipment, too much time spent on unproductive testing operations is highly uneconomical. Hence, for a number of important baseline items, testing cannot be done on a daily basis.
In general, the three most common items a deep ultraviolet exposure station needs to be tested on include reticle blind accuracy, pre-alignment accuracy and overlay accuracy.
Using Nikon's deep ultraviolet exposure station as an example, a reticle blind accuracy check requires about half an hour. Hence, the reticle blind accuracy check is normally conducted once every three months. A pre-alignment accuracy check takes about twenty minutes while an overlay accuracy check takes about two hours. Hence, the pre-alignment and overlay accuracy checks are normally done once a month. However, such a low frequency of checking the DUV exposure station is likely to miss some errors that may affect the resulting quality of the products.
FIG. 1 shows a conventional method of checking the reticle blind accuracy. Reticle blind accuracy refers to the precision of horizontal distance between the reticle and the blind that surrounds the reticle. Adjustment of the reticle blind accuracy can be achieved by using a vernier at the center of an imaging screen. Size difference between a light-exposed region known as a shot 4 against the size of a dark image 6 produced by the projection of the DUV exposure station via the blind can be compared. Hence, the extent of too much or too little coverage of the blind when light exposure for shot 4 is made can be calculated. For a Nikon DUV exposure station, the acceptable range of error for over or under coverage falls between plus or minus two hundred micrometers.
If the blind opens too little, diffraction light will occur, leading to a colorful pattern on the scribe line of a wafer. This can lead to the formation of a particle source in the wafer.
On the other hand, if the blind opens too much, interference of the light pattern with a neighboring shot may happen, leading to incomplete pattern formation on the wafer. Furthermore, a model test key related to the manufacturing process is normally formed somewhere just outside an edge of the shot and hence may be over-exposed. Once the key is over-exposed, it no longer can function as a test mark in the manufacturing process.
Before the silicon wafer is exposed to light inside a DUV exposure station, the wafer must first move to a designated position so that the DUV exposure station can automatically look for an alignment mark. The designated position is located by a pre-alignment mark. This pre-alignment mark and the alignment mark for exposure are separated by a small distance. When the distance of separation is greater than a pre-assigned value, such as 25 micrometers, the DUV exposure station can no longer find the alignment mark for exposure. In other words, the alignment mark needs to be found by a manually controlled lever system.
Should anything abnormal occur to the pre-alignment mark during the earlier stages of circuit layer fabrication, the entire wafer has to be scrapped. Even when inaccuracy of the pre-alignment mark occurs much later, alignment of the DUV exposure station for forming the next layer is likely to be difficult. Sometimes, the station has to be shut down to fix such problems; hence, utility of the station is lowered.
Conventionally, accuracy of overlays can be achieved only by following sophisticated adjustment procedures. The procedures involve forming a first photoresist layer over the wafer, and transferring a mask pattern to the photoresist layer. A second photoresist layer is formed over the first photoresist layer, and a second mask is used to form another pattern on the second photoresist layer. Because both the first and the second photoresist layers have an alignment mark for exposure, a laser beam can be used for scanning the alignment marks. The results obtained from the laser scan can be used to adjust the overlay accuracy. The whole procedure is time consuming and its accuracy is rather low.